Leiomyoma (Uterine fibroids) is a major disease that affects women but remains shockingly understudied with few nonsurgical therapeutic options. Leiomyoma is characterized by excessive deposition of extracellular matrix and enhanced proliferation and tumorigenesis of uterine smooth muscle cells. The roles of estrogen and progesterone and their nuclear receptors (NRs) and transforming growth factor ???TGF???signaling in leiomyoma are well established. However, the broader role of the nuclear receptor superfamily and their cross talk with TGF? signaling pathways in leiomyoma remain as some of the major unanswered questions. The long-term goal of this project is to establish systematically the roles of nuclear receptors (NRs) and their ligands and to understand how TGF? signaling is altered in leiomyoma. The goal of this project is to determine the novel roles of NR4A subfamily members, including NR4A1 (NGF1B), NR4A2 (NURR1), and NR4A3 (NOR1) in leiomyoma. We performed expression profiling of leiomyoma tissues and adjacent normal myometrium for all 48 human NRs and demonstrated severe deficiency of the NR4A subfamily members, including NR4A1 (NGF1B), NR4A2 (NURR1), and NR4A3 (NOR1), in leiomyoma. Our preliminary results show that NR4A functions by regulating expression of key profibrotic factors such as TGF?3 and SMAD3 and key extracellular matrix components such as collagen 1A1. Furthermore, NR4As regulate proliferation of leiomyoma primary smooth muscle cells. We hypothesize that the NR4A family of nuclear receptors plays critical and integrative roles involving TGF?? and SMAD signaling in leiomyoma development by regulating key proliferation and profibrotic genes. The specific aims are: Specific Aim 1: Determine how NR4A and TGF?? /SMAD signaling pathways interact to regulate pro-fibrotic genes in leiomyoma. Specific Aim 2: Determine the roles of NR4As and their selective modulators in leiomyoma cell proliferation in vitro and tumorigenesis in vivo. The proposed aims will advance our knowledge of this highly prevalent public health challenge for which treatment options are currently limited at best. The proposed work is scientifically, translationally, and clinically significant because it provides a new perspective on and better understanding of the mechanisms underlying leiomyoma development and opens up possibilities for identifying additional therapeutic targets and strategies. It is innovative because it represents the first systematic exploration of previously unrealized roles of the NR4A nuclear receptor family in leiomyoma biology.